Process for Producing Magnetic Powder and Process for Producing Dust Core

ABSTRACT

A process for producing a magnetic powder which is sufficiently reduced in core losses such as iron loss and hysteresis loss and has sufficient strength; and a process for producing a dust core. The process for magnetic powder production comprises using a magnetic-material powder produced by water atomization as a raw powder and subjecting the powder to spheroidizing in which a mechanical impact is applied to the powder to spheroidize the powder particles. After the spheroidizing, the powder is subjected to a grain enlarging treatment in which the powder is annealed at a temperature not lower than the austenite transformation point. The process for dust core production comprises compacting the magnetic powder thus produced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national phase application filed under 35 U.S.C. 371 of PCT/JP2007/060886 filed on May 29, 2007, which claims the benefit of priority from the prior Japanese Patent Application No. 2006-151201 filed on May 31, 2006, the entire contents of all of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of producing a dust core to be used in a rotary electric machine and a method of producing magnetic powder to be used as a material of the dust core and, more particularly, to a method of producing magnetic powder for producing a dust core with small core losses and a method of producing the dust core.

BACKGROUND ART

Heretofore, a dust core made of magnetic metal powder by compacting has been utilized in a rotary electric machine. As a method of producing this magnetic metal powder to be used for the dust core, for example, an atomizing method such as a water atomizing method and a gas atomizing method is useful (e.g. see Patent Documents 1 and 2).

Patent Document 1: JP8 (1996)-37107A

Patent Document 2: JP7 (1995)-245209A

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, the magnetic metal powder produced by the above conventional water atomizing method tends to have an irregular shape and many vertexes and ridges. This leads to a disadvantage that the compacted core has a high iron loss value. According to the water atomizing method, furthermore, oxide will be formed in a thick layer on the surface of particles. It is therefore difficult to reduce carbon in the powder, resulting in that the core has a high hysteresis loss.

On the other hand, the gas atomizing method can produce powder having a more nearly spherical shape as compared with that produced by the water atomizing method. Accordingly, a core having a good iron loss value can be produced. Meanwhile, the powder particles have a smooth shape and hence they are relatively weakly bonded together even after compaction. This makes it difficult to produce a core having sufficient strength. In a motor stator, for example, a plurality of cores is arranged in a ring shape and bound from outside by shrink fitting or the like. Such stator is further subjected to thermal shock cycles. In the case of using the core made of the powder produced by the gas atomizing method, consequently, there is a problem that may cause breakage in an edge portion or cracks. Furthermore, the gas atomizing method is an expensive production method, which would be less adopted as a method of producing a large number of cores.

The present invention has been made to solve the above problems in the conventional dust core producing method. Specifically, the present invention has a purpose to provide a method of producing magnetic powder and a method of producing a dust core having sufficient small core losses such as iron loss and hysteresis loss and having sufficient strength.

Means for Solving the Problems

To achieve the above purpose, the present invention provides, a method of producing magnetic powder by a water atomizing method, wherein the magnetic powder produced by water atomization is used as raw powder, and a spheroidization treatment is performed by applying mechanical impact on the powder to spheroidize a shape of the powder.

According to the magnetic powder producing method of the invention, the magnetic powder made by the water atomizing method is applied with the mechanical impact for spheroidization. Thus, largely irregular particles made by the water atomizing method are spheroidized; however, a spheroidization degree by this method is moderate. The magnetic powder produced by this producing method therefore has somewhat different shapes. The core made of this powder by compaction will have sufficient strength.

According to the present invention, preferably, a grain enlarging treatment is performed by annealing the powder at a temperature equal to or higher than an austenite transformation point after the spheroidization treatment. This enlarges the crystal grain and reduces the amount of carbon in the powder. Accordingly, the use of this magnetic powder allows production of a core having sufficiently low core losses.

As another aspect, the present invention provides a method of producing a dust core by compacting magnetic powder, wherein the magnetic powder produced by water atomization is used as raw powder, and a spheroidization treatment is performed by applying mechanical impact on the powder before compacting to spheroidize a shape of the powder.

In the dust core producing method of the invention, preferably, a grain enlarging treatment is performed by annealing the powder at a temperature equal to or higher than an austenite transformation point after the spheroidization treatment but before the compacting.

According to the magnetic powder producing method and the dust core producing method of the invention, the dust core produced has sufficiently small core losses such as iron loss and hysteresis loss and sufficiently high strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing an example of the shape of a dust core in a preferred embodiment;

FIG. 2 is a flowchart showing a dust core producing method in the embodiment;

FIG. 3 is an enlarged view of powder obtained by a water atomizing method;

FIG. 4 is an enlarged view of the powder obtained by the water atomizing method and then subjected to a jet mill treatment and an annealing treatment;

FIG. 5 is a sectional view of the powder showing an example of a crystal grain boundary and a powder circumference;

FIG. 6 is a graph to explain a difference in spheroidized shape according to a jet mill treatment time;

FIG. 7 is a graph to explain a difference in crystal grain diameter between presence and absence of the jet mill treatment and the annealing treatment;

FIG. 8 is a graph to show changes in the quantity of carbon in the jet mill treatment and the annealing treatment;

FIG. 9 is a graph to explain a difference in hysteresis loss between powder treatment methods;

FIG. 10 is an explanatory view showing a radial crushing test method; and

FIG. 11 is a graph to show a difference in strength between the powder treatment methods.

EXPLANATION OF REFERENCE CODES

-   -   20 Dust core     -   Step (2) Jet mill treatment step     -   Step (3) Annealing treatment step

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description of a preferred embodiment of the present invention will now be given referring to the accompanying drawings. In the present embodiment, the invention is applied to a dust core made of magnetic powder by compaction.

FIG. 1 shows an example of the shape of a dust core in this embodiment. This dust core 20 is made of magnetic metal powder by compaction using a die to integrally form a tooth 21 and a yoke 22. The tooth 21 is a part on which a coil will be wound in concentrated winding to function as a core. In the invention, the dust core may have any shape.

The dust core producing method of this embodiment is, as shown in FIG. 2, achieved by the following six steps in order:

-   -   (1) Production of raw powder;     -   (2) Jet mill treatment;     -   (3) Annealing treatment;     -   (4) Coating treatment;     -   (5) Compacting;     -   (6) Heating treatment.

In the raw powder producing step (1), firstly, raw magnetic metal is powderized by the water atomizing method. The raw magnetic metal is preferably an Fe—Si material containing 1% or more Si. The powder obtained by the water atomizing method contains variously shaped particles as schematically shown in FIG. 3. They are also largely different in size. Furthermore, an oxide layer is formed on surface of particles.

In the next jet mill treatment step (2), the powder obtained in (1) is subjected to the jet mill treatment or may be subjected to a ball mill treatment. Accordingly, the convex portions are chipped away from each powder particle, which is slightly spheroidized, and simultaneously the oxide layer is removed from the surface.

In the annealing treatment step (3), successively, the annealing treatment is performed. An annealing temperature in this treatment is preferably determined to be about 980° C. or more at which austenite transformation occurs. Vacuum annealing is more preferable. “Vacuum” in this vacuum atomizing treatment does not mean so-called ultrahigh vacuum but represents a state where the pressure is reduced to a certain level. FIG. 4 schematically shows the powder obtained after the jet mill treatment (2) and the annealing treatment (3) applied to the water-atomized powder obtained in (1). As shown in FIG. 4, the powder having been subjected to the annealing treatment is rather nearly spherical as compared with the powder as water-atomized shown in FIG. 3 and the crystal grain diameter of the powder is also enlarged. As mentioned later, it is more decarburized than before the annealing.

From observation of a cross section of each powder particle, as schematically shown in FIG. 5, a crystal grain boundary (a dotted line L1 in FIG. 5) was found inside the powder. Specifically, several crystal grains bond together into one particle. The inventors of the present invention therefore carried out image analysis by photographing or imaging a cross section of the powder and compared the following two values in order to compare the powders produced by different producing methods.

Spheroidization degree=Powder circumferential length/Powder area

Crystal grain degree=Crystal grain boundary length/Crystal grain area

Here, the “Powder circumferential length” represents the length of an outer circumference of a particle in a sectional view (the length indicated by a bold line L2 in FIG. 5). The area within the outer circumference represents the “Powder area”. As a value of the “Spheroidization degree” is smaller in the above expression, the shape of a particle is closer to a spherical shape. Furthermore, “Crystal grain boundary length” represents the length of an outer circumference of a crystal grain, i.e. the length of the crystal grain boundary L1 and an outer circumference surrounding one grain, in the sectional view. The area surrounded by the crystal grain boundary and the outer circumference represents the “Crystal grain area”. As a value defined by the above expression is smaller, the crystal grain diameter is larger, which shows that the grains are enlarged. In this embodiment, by the treatments (2) and (3), the powder closer to a spherical shape and including enlarged crystal grains could be obtained.

In the coating treatment step (4), thereafter, the powder obtained in (3) is subjected to silicone resin coating. In the compacting step (5), the thus obtained magnetic metal powder is compacted by use of a die. In the heating treatment step (6), subsequently, heating is performed. A temperature in this treatment is preferably determined to be 750° C. or less. If it is higher than it, an SiO₂ coated layer generated from silicone resin in the coating treatment step (4) may be broken. The dust core producing steps are completed as above. The step (1) corresponds to the conventional water atomizing method. The steps (4) to (6) are the same as those conventionally performed in general for dust core production.

According to the producing method in this embodiment, the water-atomized powder having an irregular shape and a largely different size is subjected to the jet mill treatment and the annealing treatment before the coating treatment. Accordingly, the powder particles are spheroidized and the crystal grain diameter is also enlarged. Furthermore, the amount of carbon contained in the powder is reduced in the annealing treatment. The dust core molded by use of such powder can have a reduced hysteresis loss. On the other hand, in the jet mill treatment (2), the powder could not be spheroidized than in the gas atomizing method and thus the powder remains irregular in shape to some extent. Consequently, in the dust core molded by using this powder, the particles are strongly bonded together to provide sufficient strength.

EXAMPLE

An example of the present embodiment is explained below. In this example, a Fe—Si material was used as raw metal and it was powderized by the water atomizing method (step (1)). The particle diameter of the powder was about 75 to 350 μm. With a jet mill made by NPK Corporation, the jet mill treatment was conducted under condition of about 0.6 MPa of air pressure (step (2)). This treatment time is preferably 30 min or longer and 60 min or shorter.

A cross section of the powder produced as above was photographed and image-analyzed to calculate the spheroidization degree (the powder circumferential length/the power area) of the powder. The spheroidization degree was compared according to the jet mill treatment time. The result thereof is shown in FIG. 6. In FIG. 6, a solid line indicates a change in spheroidization degree according to the jet mill treatment time in the present example. In FIG. 6, a lower powder is closer to a spherical shape than others. As seen from this figure, the spheroidization degree of the powder having been subjected to no jet mill treatment was about 0.053, whereas the powder having been subjected to the jet mill treatment for 60 min. was about 0.044.

In other words, it is found that the powder is spheroidized by the jet mill treatment. In this figure, a broken line indicates the spheroidization degree (about 0.04) of the powder produced by the gas atomizing method. The powder obtained by the water atomizing method and the jet mill treatment was not spheroidized than the powder obtained by the gas atomizing method.

Furthermore, the powder having been subjected to the jet mill treatment was subjected to the annealing treatment (step (3)). In this example, the annealing treatment was conducted under vacuum at 1100° C. for 3 h. Thus, the crystal grain was enlarged as shown in FIG. 7. Here, the cross sections of the particles produced respectively were photographed and image-analyzed. Based on the result thereof, the crystal grain degree (Crystal grain boundary length/Crystal grain area) was calculated. In FIG. 7, a lower one represents a larger crystal grain. To be specific, the powder having been water-atomized and then annealed has a larger grain size than the powder as water-atomized. When the powder was additionally subjected to the jet mill treatment before the annealing treatment, the crystal grain diameter was further increased.

A change in the amount of carbon contained in the powder subjected to the jet mill treatment and the annealing treatment was examined. The amount of carbon contained in the water-atomized powder itself is about 0.014 wt %. The amount of carbon in the powder after the jet mill treatment and the annealing treatment was examined by changing the jet mill treatment time. The result thereof is shown in FIG. 8. That is, the case where only the vacuum annealing treatment was conducted without the jet mill treatment (corresponding to the jet mill treatment time: 0 min. in the figure), the amount of carbon decreased to about 0.0045 wt %. Furthermore, when the jet mill treatment was carried out for 30 min. or more and then the vacuum annealing treatment was performed, the amount of carbon decreased to about 0.0013 wt %.

In the coating treatment step (4), 0.2 to 0.5 wt % of silicone resin was added, agitated, and dried. In the compaction step (5), molding was conducted with surface pressure of 1200 to 1600 MPa by a warm die wall lubricating compaction method. In the heating treatment step (6), successively, the heating treatment was performed in a nitrogen atmosphere at 600 to 750° C. for 30 min. The dust core of the present example was produced as above.

COMPARISON BETWEEN THE PRESENT EXAMPLE AND A COMPARATIVE EXAMPLE

A comparative examination between the present example and various comparative examples was performed. Firstly, hysteresis loss was compared between test pieces made of four kinds of powders, namely, the present example in which the water-atomized powder has been subjected to the jet mill treatment and the annealing treatment and three comparative examples, i.e., the gas atomized powder, the water-atomized powder as atomized, and the water-atomized powder subjected only to the jet mill treatment. Accordingly, the above steps (4) to (6) were performed by using those four kinds of powders to produce annular test pieces T as shown in FIG. 10. Herein, each test piece T was made with an outer diameter of 39 mm, an inner diameter of 30 mm, and a thickness of 5 mm.

On each test piece T, an excitation coil and a detection coil were wound. BH curve was measured by a direct-current BH analyzer to measure hysteresis loss. The result thereof is shown in FIG. 9. The test piece T made of the powder of the present example exhibited the second lowest hysteresis loss following the test piece T made of the gas atomized powder. That value was not problematic in use.

Next, various kinds of test pieces T were compared in strength. As in the above, the test pieces T were produced by using the powder of the present embodiment, the gas atomized powder and the water-atomized powder as atomized for comparison. They were subjected to a strength test. As the strength test, a radial crushing test was performed by placing each test piece T vertically on a flat plate as shown in FIG. 10, applying pressure on each test piece T in a diametrical direction, and measuring the pressure at which breakage was caused. The result thereof is shown in FIG. 11. The test piece T made of the powder of the present example exhibits the strength nearly equal to that made of the powder as water-atomized. That value was not problematic in use.

For instance, a motor core may be manufactured in such a way that a plurality of dust cores each having the shape as shown in FIG. 1 is arranged in a ring shape and bound by shrink fitting or the like. When the dust cores are bound in such ring shape, maximum stress occurs in the boundary between the yoke and the teeth of each dust core. Accordingly, there is a risk that a dust core having low strength is cracked in such portions. Such dust core needs to be increased in strength in consideration of the thermal shock cycle which will be added when the dust core is mounted in a motor and operated actually. In the case of a dust core insufficient in strength, an edge portion may be broken or chipped when the dust core is bound. According to the producing method of the present embodiment, a dust core sufficiently large strength can be produced without causing the above defects.

In the dust core producing method of the present embodiment, as explained above in detail, the water-atomized powder applied with the jet mill treatment and then the annealing treatment is used as raw powder. The jet mill treatment increases a spheroidization degree of particles of magnetic powder. In addition, the annealing treatment enlarges the crystal grain and decreases the amount of carbon. By those treatments, accordingly, a dust core with sufficiently low hysteresis loss can be produced. Moreover, the spheroidization degree of each particle will not excessively increase and thus the dust core having sufficient strength can be produced. Consequently, the producing method of dust core with sufficiently low hysteresis loss and sufficiently high strength can be provided.

The above embodiment merely shows an example and does not give any limitation to the present invention. The present invention therefore may be embodied in other specific forms without departing from the essential characteristics thereof. For instance, the shape of the dust core in the figure is an example and the invention is not limited thereto. The term “water” in the water atomizing method and the water-atomized powder is not limited to pure water but may appropriately contain a mixture generally used in the atomizing method. Moreover, the annealing treatment may be conducted in an inert atmosphere such as nitrogen instead of being conducted under vacuum. 

1. A method of producing magnetic powder by a water atomizing method, wherein the magnetic powder produced by water atomization is used as raw powder, a spheroidization treatment is performed by applying mechanical impact on the powder to spheroidize a shape of the powder, and after the spheroidization treatment, a grain enlarging and decarburizing treatment is performed by annealing the powder at a temperature equal to or higher than an austenite transformation point to enlarge grains and reduce an amount of carbon.
 2. The method producing magnetic powder according to claim 1, wherein a coating treatment is performed by coating silicone resin on a surface of the powder to form a silicon oxide layer after the grain enlarging and decarburizing treatment.
 3. A method of producing a dust core by compacting magnetic powder, wherein the magnetic powder produced by water atomization is used as raw powder, a spheroidization treatment is performed by applying mechanical impact on the powder to spheroidize a shape of the powder, and after the spheroidization treatment but before compacting, a grain enlarging and decarburizing treatment is performed by annealing the powder at a temperature equal to or higher than an austenite transformation point to enlarge grains and reduce an amount of carbon.
 4. The method producing magnetic powder according to claim 3, wherein a coating treatment is performed by coating silicone resin on a surface of the powder to form a silicon oxide layer after the grain enlarging and decarburizing treatment but before the compacting. 